This invention relates to refractory fiber compositions and more particularly to a material for use at an intermediate temperature range as thermal insulation.
Aluminum silicate fibers containing essentially no ingredients except alumina and silica have been widely used for many years for thermal insulation capable of withstanding temperatures up to approximately 1260.degree. C. Aluminum silicate fibers containing certain additive materials such as chromia and borax have been used for temperatures up to and in excess of 1315.degree. C. Such compositions are difficult to form into fibers since the melting temperature is high and the temperature range in which the melt remains in the proper condition is narrow and solidification is rapid. These fibers are therefore quite expensive and are used only where the high temperature ranges involved are required.
In the temperature range below about 650.degree. C., there are a wide variety of inexpensive fiber materials which can be used as insulation. Examples are glass fibers and mineral wools. These inexpensive materials, however, are very limited as to the maximum temperature which they will withstand. Between the high temperature, expensive fibers previously mentioned, and the inexpensive, low temperature fibers there is a gap. Thus, where temperature requirements call for insulation which is serviceable in the temperature range of 650.degree. C.-1100.degree. C. either the expensive fibers or some kind of refractory block insulation must be used.
The service temperature of fiber insulation is determined by three criteria. The first is the fact that the fiber must not melt or sinter at the service temperature. The second criteria is that the blanket made from the fibers must not have excessive shrinkage at the service temperature. Excessive shrinkage causes openings to form between the blankets through which the heat can flow. The third criteria is the amount of devitrification or crystallization that occurs. The more crystalline the fiber becomes, the more brittle it is, thus decreasing the flexibility and resiliency of the blankets.
It is thus evident that there is a significant need for inexpensive ceramic fiber materials which will have service temperatures in the range of 650.degree. C. to 1100.degree. C. Such fiber would find widespread use in bulk, mat or blanket form as insulation for furnaces, kilns and the like wherein the insulation is subjected to temperatures in this range. They would also be particularly useful as insulations for catalytic mufflers.